Cooling systems remove waste heat from industrial processes through a heat transfer mechanism. Since water is the medium for removing heat from the system, metal parts in the cooling system can become corroded. Such metal parts in the cooling system may include chiller systems, heat exchangers, auxiliary equipment and system piping.
Corrosion of metal parts results from the oxidation of the metal when exposed to an oxidizing compound. Corrosion is an electrochemical process in which a difference in electrical potential (voltage)develops between two metals or between different parts of a single metal. This potential can be measured by connecting the metal to a standard electrode and determining the voltage. The potential generated can be expressed as positive or negative. A corrosion cell is then produced in which the current passing through the metal causes reactions at the anode (area of lower potential) and cathode (area of higher potential).
The following shows the sequence of events as metal becomes oxidized: (1) Fe.sup.0 is lost from the anode to the bulk water solution and becomes oxidized to Fe.sup.2+. (2) Two electrons are released through the metal to the cathode. (3) Oxygen in the water solution moves to the cathode and forms hydroxyl ions at the surface of the metal producing ferrous hydroxide.
Ferrous hydroxide precipitates quickly on the metal surface as a white floc and is further oxidized to ferric hydroxide. When these reaction products remain at the cathode, a barrier is formed that physically separates the O.sub.2 in the water from the electrons at the metal surface. This process is called polarization and protects the metal from further corrosion by minimizing the potential between the anode and the cathode. Removal of this barrier, called depolarization, through lowering of the pH or by increasing the velocity of the water produces further metal oxidation and the detrimental corrosion products of ferric or iron oxide, and rust.
Prefilming or passivation of equipment is a common practice in extending the life of equipment in aqueous systems. When equipment is new, a chemical corrosion inhibitor is added initially to form an impervious film to halt corrosion. Once the protective film is formed, a small amount of a corrosion inhibitor is continuously required to maintain the film and inhibit corrosion. However, changes in a cooling system environment such as low pH excursions, process leakage, microbiological deposition, organic and inorganic fouling can cause disruption and penetration of the protective film allowing production of corrosion products.
The corrosion can manifest itself in various forms such as uniform attack, pitting or tuberculation to name a few. Significant amounts of rust reduce heat transfer efficiency and can accelerate corrosion rates by the formation of concentration cells under the corrosion deposit. This can negatively affect the overall operation of a cooling system resulting in reduced operating efficiency, increased maintenance costs and down time as well as shortened equipment life. Once iron oxide is present in significant amounts, cleaning of the equipment to remove the corrosion products is necessary.
The current practice for years in iron oxide removal was to shut down the system and add an acid cleaner containing hydrochloric, sulfuric, sulfamic, gluconic or citric acids, reducing the pH to 3.0 to 3.5, and circulating the solution for several hours with heat. This process can be very corrosive to the base metal of equipment causing increased metal loss once the iron oxide is removed. Holes in the metal of critical equipment can be created quickly, resulting in process leakage and/or reduced operating efficiency. In addition, the handling of large amounts of strong acids can be hazardous for plant employees. Another method for removing corrosion from metals exposed to an aqueous system, is to circulate high concentrations of a chelant like ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) to sequester and bind iron. This can be cost-prohibitive since it can result in large amounts of chelant consumed in heavily fouled systems as it functions stoichiometrically.
Just recently, several neutral-type on and off-line treatments were brought to the marketplace. These methods usually involve a much longer treatment time and may utilize tannins or similar-type compounds which can ultimately be used by microbes as a nutrient source creating a deposition problem. These compounds generally have only a 50% rate of conversion of insoluble Fe.sup.3+ to a more soluble form, Fe.sup.2+ resulting in less than efficient cleaning. Moreover, a neutralizer or acid addition step requiring additional chemical cost and handling is generally necessary with the neutral cleaners to aid in iron oxide removal and pH control.
U.S. Pat. No. 3,527,609 discloses a two stage method of removing iron oxide: (1) adding an alkali metal salt or ammonium salt of amino polycarboxylic acid to a recirculating system while adjusting pH to 8-11 then (2) acidifying system water to pH to 4-5.5 with sulfuric acid to remove iron oxide. U.S. Pat. No. 5,466,297 explains a method for removing iron oxide and recycling ferrous/ferric compounds with the use of a citric acid-tannin and erythorbic acid blend while adjusting the pH of the cooling water system to a range of 1-5. Canadian Patent 1,160,034 teaches a method of removing iron oxide by adding 3-300 ppm of a sulfated glyceryl trioleate and 2-hepto-1-(ethoxy propionic acid)imidazoline into an acid cleaner. The multi-component product is then applied to maintain a pH of 1-6 to clean rust and other deposits in a cooling system.